$A$ boy runs and sits on a rotating platform. Which of the following quantities will be conserved?

$A$ circular disc of mass $M$ has an initial angular velocity $\omega_1$. Two small spheres,each of mass $m$,are attached to the disc at opposite points on its diameter. What will be the final angular velocity of the disc?

$M$ द्रव्यमान और $R$ त्रिज्या वाली एक डिस्क $x-y$ तल में चित्रानुसार गति कर रही है। दिखाए गए क्षण पर डिस्क का कोणीय संवेग क्या है?

$A$ thin uniform circular disc of mass $M$ and radius $R$ is rotating with angular velocity $\omega$ in a horizontal plane about an axis passing through its centre and perpendicular to its plane. Another disc of the same radius but of mass $\frac{M}{3}$ is placed gently on the first disc co-axially. The new angular velocity will be:

Suppose all the people in the world line up at the equator and all start running at speed $v_{rel}$ relative to the surface of the Earth along the equatorial circle. The initial angular velocity of the Earth is $\omega_0$. Let the moment of inertia of the Earth be $I_E$,the moment of inertia of all people be $I_P$,and the radius of the Earth be $R$. Which of the following statements is correct?

If no external torque acts on a system,which of the following remains constant in both magnitude and direction?